I've done my share of weddings and never had a problem. The newleyweds and families have been welcoming and wonderful.
That said, if it didn't specify it in your contract, then frankly it's their problem if they don't like the color of your stock keyboard. (Now if you had a bunch of stickers on it, that would be another story...)
Maybe it's me - I have no issue covering up the manufacturer's name on my board, if someone made that big a deal about the color of my keyboard I'd probably just pack up and leave... and let them do without keys. Life is way too short to deal with absurd, unnecessary drama.
As they say, that's just my .02 cents.